The Russians since NATO troops in Europe will be better supplied relative to Russian troops in a hypothetical total war (even with Chinese intervention a la Korean War) due to the Western Democracies (excluding Japan and South Korea due to virtually guaranteed buck passing) possessing greater latent power by virtue of their extensive industrial infrastructure (although somewhat weakened with the advent of deindustrialization) and more developed administrative apparatus (again, relative to Russia). A WW3 scenario would almost certainly be a total war where the the Great Powers would mobilize their entire economy for war, which would put Russia at a severe disadvantage logistically, thus their conscripts will suffer higher casualty rates relative to US conscripts.
China, on the other hand, possess a rapidly modernizing army and soon to be blue water Navy in spite of the political purges of military personnel carried out by Xi Jinping due to allegations of corruption. Despite suffering from some economic headwinds, their economy still grows at almost double the rate of the US GDP in terms of PPP (5%> and 2.5% respectively) and, in my personal opinion, China's economic headwinds (real estate troubles, persistent deflation, demographic slowdown, spiralling local government debt, high youth unemployment, and low consumption) are not as serious or insurmountable as most Western analysts think, and are transient troubles that are part of China's growing troubles as they shift to another growth model. Due to possessing a dynamic, fast growing economy (superior industrial capacity due to hollowing out of American manufacturing); a rapidly modernizing military that will soon reach parity with the US; and a potential war being located right in China's backyard (despite possessing military bases in East Asia, the US is thousands of miles away and separated by the world's largest ocean), US conscripts will suffer disportionate casualties relative to Chinese conscripts in a hypothetical total war
Thus, I would prefer fighting the Russians assuming that both scenarios are a total war and I am an ordinary grunt soldier (however, the brilliant Professor Mearsheimer has given a highly convincing counterargument that any future potential war against China will be a limited engagement relative to the pior World Wars which I will not expand on here due to laziness)
P.S. I apologize for any grammar errors since I texted this all on my phone with no proofreading